1. Field of the Invention
This invention relates to an evaporative emission control system for internal combustion engines, which suppresses emission of evaporative fuel generated in the fuel tank of the engine, into the atmosphere during refueling.
2. Prior Art
An evaporative emission control system of this kind is conventionally known, for example, from U.S. Pat. No. 4,714,172. FIG. 1 shows the arrangement of the evaporative emission control system according to U.S. Pat. No. 4,714,172. As shown in the figure, a fuel tank T is comprised of a tank main body 101, and a filler pipe 102. The evaporative emission control system includes a canister 114 for adsorbing evaporative fuel generated in the tank main body 101, an evaporative fuel passage 111 extending between the tank main body 101 and the canister 114, and a differential pressure-operated valve 107 which is formed by a diaphragm valve and arranged across the evaporative fuel passage 111. The differential pressure-operated valve 107 has a diaphragm 108 partitioning the interior of the valve housing into a back pressure chamber 107a and a positive pressure chamber 107b, a coiled spring 110 arranged within the back pressure chamber 107a, for biasing the diaphragm 108 toward the positive pressure chamber 107b, and a valve element 109 coupled to the diaphragm 108, which is displaceable together with the diaphragm 108 in response to a pressure difference between the two chambers 107a, 107b for opening and closing an opposed open end of an intake passage portion 112 of the evaporative fuel passage 111 on the canister side. A pressure-introducing passage 106 extends between the back pressure chamber 107a and an inlet end portion 104 of the filler pipe 102.
During refueling of the fuel tank T, atmospheric pressure is introduced into the inlet end portion 104 of the filler pipe 102. Due to pouring of fresh fuel into the filler pipe 102, the atmospheric pressure is transmitted through the pressure-introducing passage 106 into the back pressure chamber 107a of the differential pressure-operated valve 107 to open the valve 107, whereby evaporative fuel generated in the tank main body 101 is delivered through the evaporative fuel passage 111 to the canister 114 to thereby prevent evaporative fuel from being emitted into the atmosphere.
However, since during refueling evaporative fuel is generated in large quantities, which is delivered to the canister 114, the canister 114 has to be designed to have a large capacity, which leads to an increased manufacturing cost.
One way to eliminate the above inconvenience is to provide another evaporative fuel passage which extends between the tank main body 101 and the inlet end portion 104 of the filler pipe 102 to return part of evaporative fuel generated in the tank main body 101 to the filler pipe 102.
However, the employment of two evaporative fuel passages necessitates the use of an increased number of passage component parts and complicates the layout and piping, also resulting in an increased manufacturing cost.